1. FIELD OF THE INVENTION
The present invention relates to laser processing of materials, and more specifically to a method and apparatus for more efficiently and precisely controlling and steering a laser beam for welding, cutting, machining, and marking materials.
2. DESCRIPTION OF THE PRIOR ART
The use of laser beams for welding, fusing, cutting, machining, marking, and other processing of materials is well known, although there have heretofore been some persistent, long-recognized problems that have inhibited the efficiency and cost-effectiveness of such laser beam processing for many practical applications. Since welding, fusing, cutting, machining, or marking all involve focusing the energy of a laser beam on the material being processed with sufficient power to melt the material, the single term "welding" is used for convenience in this discussion to include any or all of these material processing techniques, even though technically welding usually refers to joining work pieces together.
One of these persistent problems in laser welding has been energy losses due to reflection of the laser beam on the surface being welded. To minimize such losses due to reflection, it has become common and accepted practice to keep the laser beam oriented normal to the plane of the work piece being welded whenever possible. In other words, the angle of incidence is kept at or very near to 0.degree.. Consequently, when the weld is to be linear, i.e., not merely a spot on the work piece surface, either the laser beam has to be moved over the work piece surface while maintaining the normal angle of incidence, or the work piece has to be moved under a stationary laser beam.
Since common industrial laser generators with enough power to weld are quite large and require substantial cooling apparatus, precision mounting and moving the laser generator is difficult and expensive. Therefore, most practitioners and industrial applications opt for moving the work piece under a stationary laser beam that is kept normal or near normal to the surface of the work piece to be welded. These features are common to the methods and apparatus disclosed in U.S. Pat. No. 4,492,843, issued to R. Miller, et al.; U.S. Pat. No. 4,471,204, issued to H. Takafuji, et. al., U.S. Pat. No. 4,326,118, issued to J. Smith, Japanese Pat. No. 53-48040, issued to Mitsubishi Electric Corp.; and Japanese Pat. No. 54-100948, and issued to Kawasaki Steel KK.
However, moving the work piece can create substantial problems as well. Some work pieces are large and difficult to handle. Also, many industrial and experimental processes are such that fastening and moving the work piece is impractical for other reasons. For example, some welding processes have to be done in vacuum chambers where enclosing precisely moveable work piece tables and changing work pieces in such vacuum chambers would be difficult and very expensive. Some work pieces may have many individual, perhaps even fragile, components to be welded in configurations where clamping and holding them in proper positions for welding is difficult, and movement prior to welding could displace them. Further, it has been found that in some circumstances, such as welding the edges of glass plates together, clamping the plates in position, welding, and then cooling in the clamped position creates internal stresses that annealing cannot eliminate, thus causing cracks in the finished material.
Consequently, there is a real need for being able to keep both the laser generator and the work piece stationary during welding. Some attempts to do so have included methods and apparatus for moving the laser beam itself over the work piece while maintaining both the laser generator and the work piece stationary. For example, U.S. Pat. No. 4,578,554, issued to L. Coulter, discloses the use of optical fibers to guide and move the laser beam over the work piece. U.S. Pat. No. 4,125,755 discloses a method of stacking work pieces in predetermined positions in relation to a stationary laser generator so that horizontal translation of a reflecting mirror while keeping a constant reflective angle moves the laser beam from point to point while still maintaining the vertical laser beam orientation and constant focus on the points to be welded. U.S. Pat. No. 3,594,532, issued to F. Lunan, et al., discloses an apparatus that moves the optical reflector and lens assembly in relation to a stationary laser generator and a stationary work piece in a manner that keeps the incident laser beam normal to the surface of the work piece.
Another significant, well-known problem of long duration in laser welding, and here perhaps more specific and significant to laser cutting, is the variations in kerfs due to polarization of the laser beam, as illustrated in FIGS. 1a-1d. The laser beam initially generated by a laser source is coherent, and possibly linearly polarized electromagnetic radiation. Electromagnetic radiation is propagated in waves comprised of an electric field E component and a magnetic field H component perpendicular to each other. In linearly polarized electromagnetic radiation, the E field components of all the wave fronts in the beam are directionally aligned with each other, i.e., the electric fields oscillate in only one direction, which is often called the electric vector or E-vector.
It is well known that when a specimen or work piece is moved under a fixed focus laser beam 1 at normal incidence, the kerfs cut by the laser beam 1 vary in characteristics, depending on the direction of movement in relation to the electronic or E-vector orientation or direction of oscillation. In FIG. 1a, the E-vector orientation, also referred to as the plane of polarization, of the laser beam 1 is indicated by the arrow 3. When the direction of travel of the cutting laser beam 1 in relation to the work piece 4 is parallel to the E-vector or plane of polarization 3, as illustrated at location 1b in FIG. 1a, the resulting kerf 5 shown in FIG. 1b is narrow, sharp, and even with perpendicular sidewalls. This condition is optimum and is the most efficient. The greatest cutting speed with the smoothest, straightest, narrowest kerf is achieved when cutting under this condition.
However, when the direction of travel of the cutting laser beam 1 in relation to the work piece 4 is oblique to the E-vector or plane of polarization 3, as illustrated at 1c in FIG. 1a, the resulting kerf 6 shown in FIG. 1c is broader with skewed sidewalls and rough edges. This condition is not as efficient or as desirable as the condition shown in FIG. 1b.
Further, when the direction of travel of the cutting laser beam 1 in relation to the work piece 4 is perpendicular to the E-vector or plane of polarization 3, as illustrated at 1d in FIG. 1a, the resulting kerf 7 shown in FIG. 1d is very broad and straight, but rough and non-uniform. This condition, while somewhat better than that shown in FIG. 1c, is still less efficient and less desirable than that shown in FIG. 1b.
Because of the phenomena described above, it has heretofore been generally understood as stated in current literature on this subject that linearly polarized laser radiation is unacceptable for contour cutting and probably has detrimental effects in welding as well. See, e.g., Dr. James T. Luxton, GMI Engineering and Management Institute, Flint, MI, "Optics for Materials Processing," in THE INDUSTRIAL LASER ANNUAL HANDBOOK, 1986 Ed., Penn Well/Laser Focus.
To overcome these problems with linearly polarized laser beams, it is common in the industry to convert linearly polarized beams to circularly polarized beams. Essentially, when electromagnetic radiation travels through a doubly refracting or birefringent crystal, two sets of wavelets propagate from every wave surface in the crystal, one set being circular and the other ellipsoidal. The rays that correspond to wave surfaces tangent to the spherical wavelets are undeviated and are commonly called the ordinary rays. The rays that correspond to wave surfaces tangent to the ellipsoids are deviated and are commonly called the extraordinary rays. When the ordinary and extraordinary rays in a doubly refracting crystal are separated, each ray taken alone is linearly polarized, but with directions of vibrations or E-vectors at right angles. However, when the crystal is cut with its faces parallel to the optic axis, so that radiation, incident normally on one of its faces, traverses the crystal in a direction perpendicular to the optic axis, the ordinary and extraordinary rays are not separated. They traverse the same path, but with different speeds. Upon emerging from the second face of the crystal, the ordinary and extraordinary rays are out of phase with each other and give rise to either elliptically polarized, circularly polarized, or linearly polarized light, depending upon a number of factors. Essentially, if the crystal has such a thickness to cause a phase difference of .pi./2 between the ordinary and extraordinary rays at the given frequency of the laser beam radiation, a circular oscillation or vibration results, and the radiation emerging from the crystal is circularly polarized.
Therefore, by insertion of an optical device such as that described above, commonly known as a quarter-wave plate, into the linearly polarized laser beam, the beam can be converted to a circularly polarized beam. Such a circularly polarized beam is equivalent for practical material processing purposes to unpolarized or randomly polarized radiation. Therefore, with such a circularly or randomly polarized laser beam, the worst or most adverse effects resulting from direction of beam travel as described above and illustrated in FIGS. 1a-1d can be avoided. However, at the same time, the optimum conditions illustrated by FIG. 1b are also sacrificed. Consequently, the result is somewhere between the best and worst conditions, which makes the cutting or welding workable for most applications, albeit not the most efficient.
There are, as mentioned above, a significant number of applications in which it is at least desirable, if not essential, to keep both the work piece and the laser generator stationary, yet to steer the laser beam in any desired direction on the work piece. As described above, the two problems encountered that have heretofore inhibited such a system are energy loss due to reflectance and unacceptable kerf variations. Previous systems have sought to minimize these effects by maintaining normal incidence to reduce reflection and converting the linearly polarized laser beam to a circularly polarized beam to reduce the kerf variations. Both of these prior art solutions are only partially effective and involve comprises. The normal incidence of the beam reduces reflection, but does not optimize or minimize reflection conditions, and the physical requirements and apparatus necessary to maintain normal beam incidence are cumbersome and confining. The circularly polarized beam avoids the worst kerf variations, but sacrifices the optimum conditions needed for the best kerfs and the most efficient cutting or welding.
Consequently, there remained prior to this invention a substantial need for a method and apparatus for laser welding that has the flexibility of a laser beam that can be steered easily in any direction over substantial areas as both the work piece and the laser generator remain stationary, yet which also minimizes reflection losses and allows the use of linearly polarized radiation for optimum efficiency and kerf production.